Although, in principle, applicable to any data transmission system, the present invention is explained on the basis of a cellular CDMA (Code Division Multiple Access) data transmission system.
CDMA-coded data transmitted via multipath channels may be detected by rake reception or methods which eliminate mutual data interference.
In particular, Code Division Multiple Access (CDMA) enables multiple data streams to be transmitted simultaneously via a common frequency band. Using spreading codes, the data stream symbols to be transmitted are converted into signal fragments of the same length as the spreading codes. The signal fragments of the data symbols corresponding to a point in time are summed up and the summed fragments are transmitted according to their time sequence, as is described in K. D. Kammeyer, Message Transmission, 2nd Edition, Information Technology Series, Teubner, Stuttgart, 1996.
The received signal fragments may be converted back to the data symbols of the data streams by despreading them with the spreading codes. If the signals are transmitted via multiple paths (as in the case of mobile telephony), it may be advantageous to despread the signals separately for all paths and, after weighting the despread signals on all paths (using the coefficients of the channel pulse response), sum them up again. A CDMA receiver which operates in this manner may be called a rake receiver.
Multipath transmission may cause mutual interference of the transmitted signals at the receiver. In contrast to the rake receiver, this interference may be eliminated in the receiver, for example by joint detection (JD), as described in A. Klein, G. K. Kaleh and P. W. Baier: “Zero Forcing and Minimum Mean-Square-Error Equalization for Multiuser Detection in Code-Division Multiple-Access Channels”, IEEE Trans. Vehic. Tech., Vol. 45 (1996), 276-287.
CDMA codes having a large spreading factor may be constructed from two codes having a smaller spreading factor, using Kronecker multiplication. A set of CDMA codes constructed in this manner may be referred as a hierarchical code set. The OVSF (Orthogonal Variable Spreading Factor) codes used in UMTS (Universal Mobile Telephone System) are hierarchically constructed codes (see Concept Group Alpha: “EVALUATION DOCUMENT (DRAFT 1.0), Part 1”, ETSI Tdoc SMG2 270/97).
The rake receiver and the joint detection method for receiving hierarchical codes may be unnecessarily complex and unnecessarily expensive.